1. Field of the Invention
The field of the present invention pertains to novel plasticized vinyl chloride polymers obtained by polymerizing vinyl chloride in the presence of polycaprolactone and methods for producing such polymers.
2. Discussion of the Background
The conventional processes for plasticizing vinyl chloride polymers (PVC) consist of homogeneously blending vinyl chloride polymers with high-boiling organic compounds. Plasticizers of this kind are called "external" plasticizers and result in undesired characteristics, such as odor, turbidity, exudation, stiffening and similar properties in the foils, films or molded parts constructed from the plasticized polymers. These drawbacks arise from the volatility of the plasticizer itself or from the tendency of the plasticizer to diffuse to the surface of the thermoplastic melt. Typical examples of such plasticizers are dioctyl phthalate (DOP), diisononyl adipate or esters of trimellitic acid (cf. "Encyclopedia of PVC", Marcel Dekker, Inc., New York, 1976). Consequently, the use of such plasticized vinyl chloride polymers in demanding applications, such as are found in the medical sector (e.g., blood bags, catheter tubes, etc.), has been problematic.
The migration tendency of these monomeric plasticizers can be diminished by employing higher molecular compounds. However, such plasticizers exhibit reduced plasticizer efficiency as a consequence of their restricted mobility in the PVC matrix. Their incorporation causes additional problems, as well (C. F. Hammer in "Polymer Blends", Academic Press, New York 1978, vol. 2). With increasing molecular weight of the plasticizer, another phenomenon becomes more noticeable - a growing incompatibility of the resultant polymer blends is usually observed, which is evident from the diminished transparency of the molded parts.
Ethylene-vinyl acetate copolymers are high molecular weight polymers which demonstrate this problem in the vinyl chloride polymer system. With a high vinyl acetate content, the molded parts are transparent but stiff; on the other hand, with a high ethylene content, plasticized but cloudy molded parts are obtained.
By introducing additional carbonyl groups in the form of terpolymers of ethylene, vinyl acetate and carbon monoxide, the compatibility can be increased and transparent materials can be obtained. However, the use of such polymers, which possess very small diffusion coefficients, require technically complicated mixing procedures with comparatively long processing times.
Alternatively, polyesters having a ratio of CH.sub.2 /COO of 4-5 to 1 are compatible with vinyl chloride polymers. Polycaprolactone (PCL) is a member of this group (J. V. Koleske in "Polymer Blends", Academic Press, New York, 1978, vol. 2). Its applicability, however, has been limited to blends with PCL concentrations of up to approximately 35%, since at higher concentrations the tendency of caprolactones to crystallize impairs the desired low temperature characteristics and the transparency (D. Hardt, C. Suling, C. Lindner, and L. Morbitzer, Angew. Chem. 94 (1982), 159-169, and R. Deanin, Journal of Vinyl Technology, March 1984, vol. 6, no. 1). Moreover, processing problems persist here, as a result of the high molecular weight of the plasticizer. Consequently, streaks and unopened fish eyes are common in pressed sheets constructed from such blends.
From the above description, it is apparent that there existed a pressing need for a plasticized vinyl chloride polymer which did not suffer from plasticizer diffusion, incompatability, and crystallinity.